The present invention relates to a positron source capable of generating a positron beam of high intensity, a method of preparing the positron source, and an automated system for supplying the positron source.
Slow positron beams have been commonly used in positron microscopes, for research in physical properties and for crystal defect evaluation of the surfaces or interfaces of semiconductors and metallic materials, and recently have become useful more and more. At present, slow positron beams are generated by emitting from positron emitters (radioisotopes), or by ejecting positrons that are generated through pair creation with a braking radiation into a moderate to be slowed down the positrons. A positron emitter is often prepared by irradiating a solid target (e.g., aluminum or boron nitride) with a beam of charged particles (e.g., protons) accelerated with a cyclotron or the like; thus a positron emitter can be generated in the solid target. A braking radiation is usually generated by irradiating a heavy metal target with an electron beam accelerated with a linear accelerator or the like.
Upon the utilization of positron beams, a strong point source for a positron emitter is required. Various approaches have been proposed for increasing the intensity of positron beams, such as the improvement in moderate efficiency and the use of a stronger positron source. As a moderate, one formed of a tungsten foil which is annealed at 2000xc2x0 C. is currently used. However, such moderate cannot achieve an efficiency of the order of 10xe2x88x924 or more. Although many efforts are being made to improve moderators, drastic and practical improvements could hardly be expected. On the other hand, for preparing a strong positron source, the use of a large-scale and expensive device is needed.
In the preparation process for a strong positron source using a solid target, there is a serious problem that heat generated during the passage of a large electric current should be removed. The process also has another problem as follows. A solid target is placed nearby a moderator for the purpose of causing to emit positrons from a positron emitter generated in the target and increasing the incident efficiency of positrons generated through pair creation with a braking radiation into the moderator. When such solid target is irradiated with an electron beam or an ion beam, the moderator sustains a radiation damage or is radioactivated by a secondary radiation other than the positrons. In order to overcome this problem, it is proposed an approach for avoiding the influence of the secondary radiation during the irradiation of the target, which comprises: irradiating a solid target at a place a distance away from a moderator thereby generating a positron emitter; transferring the irradiated solid target to the place where the moderator is placed; and ejecting a beam of positrons emitted from the positron emitter in the solid target into the moderator. However, such approach is not practical. This is because the use of a solid target usually needs a cooling device for removing heat generated as a result of the irradiation and, therefore, if a solid target is to be transferred, the system as a whole will inevitably become a large scale due to the integration of the cooling device. In the process utilizing a braking radiation generated with an electron beam, it is impossible in principle to separate a heavy metal target and a moderator. Moreover, in this process, it is necessary to automate the supply of a positron source to a positron beam-generating unit for the purpose of avoiding the harmful irradiation exposure of operators.
Under these situations, the present invention is made. That is, the object of the present invention is to provide a positron source capable of generating a positron beam of high intensity without damaging a moderator, a method of preparing the positron source, and an automated system for supplying the positron source.
The present inventors have found that the positron source can be prepared using a liquid target containing H218O [18O(H2O)] as a target for generating a positron emitter, by irradiating the liquid target with a proton beam to generate a positron emitter 18F through a 18O(p,n)18F reaction, and causing to bind the 18F onto a carbon member to trap the 18F on the carbon member. This finding leads the accomplishment of the present invention.
Therefore, the present invention provides a positron source comprising a carbon member having 18F bound onto the surface thereof. The carbon member is preferably made of graphite or glassy carbon. The carbon member preferably has a rod-like or strip-like geometry onto an end of which 18F is bound.
The present invention also provides a method of preparing a positron source comprising: irradiating a liquid target containing H218O with a beam of charged particles to generate 18F; and passing an electric current through the liquid target using a carbon member as an anode to cause to bind the 18F onto the surface of the carbon member. The liquid target may contain a small amount of natural fluorine ions, for example, by the addition of a fluoride of an alkali metal which is soluble in the liquid target and is a strong electrolyte (e.g., NaF, NaHF2 and KF).
The reason for the pre-addition of a small amount of natural fluorine ions to a liquid target [18O(H2O)] is as follows. The number of the 18F atoms generated through a nuclear reaction in the liquid target is at most 3.5xc3x971015 atoms, which corresponds to only 1.1xc3x9710xe2x88x928 g in terms of the weight of fluorine atoms. Such extremely trace amount of 18F atoms might result in insufficient current for electrodeposition. In order to prevent this problem, natural fluorine ions are added to the liquid target at a concentration of 2 xcexcg/ml so that the number of the 18F atoms becomes about 100 times greater than that without natural fluorine ions. This ensures the chemical behavior of the generated 18F as Fxe2x88x92 in an aqueous solution (a liquid target). Since the amount of the fluorine ions added is very small, it is necessary for the fluorine ions to be added to the liquid target prior to the irradiation.
In the present invention, it is preferable that the carbon member (i.e., an anode) have a rod-like or strip-like geometry and an electric current be passed through the liquid target while contacting an end surface of the carbon member with the liquid target so that the 18F is concentratedly bound onto the end surface of the carbon member. It has not been made clear yet whether the bonding of the 18F onto the surface of the carbon member is via a direct bonding between the 18F and a carbon atom in the carbon member (e.g., generation of a C-F bonding) or via intercalation of the 18F into a graphite-type crystal structure of the carbon member (i.e., formation of an intercalation compound).
The present invention also provides an automated system for supplying a positron source comprising: means for moving a container with a solution containing 18F to the position where an electric current is to be passed through the solution; means for passing an electric current through the solution at that position using a carbon member as an anode; and means for transferring the carbon member after the passage of the electric current to a positron beam-generating unit. In this system, the solution containing 16F is fed to a container placed in another room, and an electric current is then passed through the solution at that place. This system may further comprise means for recovering the solution after the passage of electric current.
The present invention further provides an automated system for supplying a positron source comprising: a rotary table for rotating a container mounted thereon; means for supplying a solution containing 18F into the container; first drive means for rotationally driving the rotary table so that the container moves between the position where the solution is to be supplied into the container and the position where an electric current is to be passed through the solution in the container; a rotary member on which a carbon member is mounted; second drive means for rotationally driving the rotary member so that the carbon member moves between the position opposed to the liquid surface of the solution in the container placed in the position where an electric current is to be passed to the solution and the position opposed to a positron source-receiving section of a positron beam-generating unit; hoisting-and-lowering means for moving the rotary member up and down; and a power supply for passing an electric current through the solution in the container using the carbon member as an anode; wherein the carbon member onto the surface of which 18F is caused to bind by passing an electric current through the solution in the container using the carbon member as an anode, is attached to the positron source-receiving section of the positron beam-generating unit.
This system may further comprise contact-detection means for detecting the contact of the carbon member with the solution in the container, which enables a precise control of the depth of the carbon member immersed in the solution. The contact-detection means may also be serve as means for detecting a micro-current passing through the solution at the instant when the carbon member is contact with the liquid surface of the solution. In the system, a plurality of containers may be mounted on the rotary table and the same numbers of carbon members as that of the containers may be mounted on the rotary member so that a continuous operation becomes possible for a long time of period.
The H218O-containing liquid target can be fed to any place readily through a pipe. Therefore, if it is possible to irradiate the H218O-containing liquid target to generate a positron emitter 18F, transfer the 18F-containing solution by remote control to the place where the positrons are used, and trap the 18F on the carbon member at that place in the state that the 18F binds onto a very small area of the carbon member, then undesirable damage of a moderator or background noise of the measurements caused by the secondary radiation during the irradiation of the liquid target can be prevented by transferring only the carbon member (i.e., the positron source) to the place where the moderator is set. In addition, by confining the surface area of the carbon source onto which the positron emitter 18F is intended to be bound within narrow limits, the density of the positron source in the surface area can be increased and, consequently, a positron beam of high intensity can be generated. According to the present invention, since the irradiation of the target is performed at a place a distance away from the moderator, the influence of the secondary radiation caused by the irradiation can be eliminated.
In the present invention, it is also preferable to immediately recover the H218O remaining in the solution after the preparation of a positron source is completed, because H218O is a very expensive material and the amount of 18O converted into 18F in one irradiation is extremely small. If the H218O is not recovered immediately and allowed to leave in the solution, it is not only evaporated as water vapor, but also normal water is dissolved into the H218O-containing solution to reduce the concentration of the H218O.
This specification includes part or all of the contents as disclosed in the specifications and/or drawings of Japanese Application Nos. 10-248611 and 10-308533, which are priority documents of the present application and incorporated herein by reference in their entirety.